1. Technical Field
This disclosure relates to reflecting wavelength plates and optical pickups having the reflecting wavelength plates and, in particular, to the reflecting wavelength plate of an optical element suitable for converting light emitted from a light source into a proper polarization state and an optical pickup using the reflecting wavelength plate.
2. Description of the Related Art
A known optical pickup is configured in the manner as shown in FIG. 1. In FIG. 1, emitted light as coherent light emitted from the light source 1 of a semiconductor laser is emitted as linear polarized light and directed toward an optical information recording medium (optical recording medium 8) as forward-traveling light. First, the emitted light is incident on a diffraction element 2 for three beams and diffracted. Thus, three forward-traveling beams are generated as the forward-traveling light and emitted toward a polarization beam splitter 3. The three forward-traveling beams are reflected by the polarization beam splitter 3 that reflects the polarization component of the forward-traveling light, so that the path of the three forward-traveling beams is bent by 90 degrees. The three forward-traveling beams are converted into parallel light by a collimating lens 4, deflected 90 degrees by a deflecting mirror 5, and incident on a (¼) wavelength plate 6. The three forward-traveling beams are converted from the linear polarized light to circular polarized light by the (¼) wavelength plate 6 and emitted to an objective lens 7. Then, the objective lens 7 causes the three forward-traveling beams to converge onto the optical recording medium 8 (forward path).
The three forward-traveling beams emitted from the objective lens 7 are incident and converged onto the recording surface of the optical recording medium 8, and then three backward-traveling beams are reflected to the objective lens 7 in the direction opposite to an incident direction by the recording surface as backward-traveling light. Thus, the main beam of the three forward-traveling beams is configured to, for example, read information from the recording surface or write information on the recording surface. Furthermore, the sub-beam of the three forward-traveling beams is used for detecting a track error signal.
The three backward-traveling beams reflected by the recording surface of the optical recording surface 8 are incident on the objective lens 7, converted into parallel light by the objective lens 7, and emitted to the (¼) wavelength plate 6. The three backward-traveling beams emitted from the objective lens 7 are incident on the (¼) wavelength plate 6, converted from the circular polarized light to the linear polarized light in the deflecting direction orthogonal to the forward-traveling light by the (¼) wavelength plate 6, and emitted to collimating lens 4. The three backward-traveling beams emitted from the (¼) wavelength plate 6 are incident on the collimating lens 4 and converted into converging light by the collimating lens 4. The three backward-traveling beams permeate the polarization beam splitter 3 that allows permeation of the light of a polarization component orthogonal to the forward-traveling light, permeate a detection lens 9, and are incident on a light receiving element 10. The three backward-traveling beams are converted into an electrical signal by the light receiving element 10 (backward path).
However, in the optical pickup having the above-described configuration, the (¼) wavelength plate 6 arranged between the objective lens 7 and the deflecting mirror 5 causes a bottleneck for reducing the thickness of the optical pickup to thin the optical pickup. Patent Document 1 discloses a technology for solving this problem.
The invention of Patent Document 1 is related to an optical pickup device that records and reproduces data on and from an optical recording medium by using laser light emitted from a semiconductor laser element. In this optical pickup device, a ( 1/7) wavelength plate (reflecting wavelength plate) that deflects light source light is arranged on an incline by approximately 45 degrees with respect to the light axis of an objective lens that converges the light source light onto the optical recording medium. This ( 1/7) wavelength plate converts linear polarized light into circular polarized light.
As described above, the light source light is deflected into the objective lens by the ( 1/7) wavelength plate, and the polarized light from the light source light is converted from the linear polarized light to the circular polarized light. This configuration allows the conventionally-required deflecting mirror 5 and the (¼) wavelength plate 6 to be used in common, whereby optical elements can be eliminated. Moreover, this configuration allows the optical pickup device to be thinner than it otherwise would be.
Meanwhile, the ( 1/7) wavelength plate of Patent Document 1 is made of a crystal plate, and the wavelength of the light source light is 790 nm. It is generally known that uniaxial anisotropy crystals such as artificial or natural rutile, calcite, and crystal can be used as the wavelength plate. However, the artificial crystal are difficult to uniformly develop, while the optically-uniform and large natural crystal is hard to obtain and expensive.
In addition, the following problems must be taken into consideration. That is, an apparatus that can be configured to reproduce optical recording media of both a DVD and a CD is known. This apparatus requires light sources that separately emit light for a DVD and light for a CD each having a different wavelength. The apparatus uses the so-called twin-beam-type light source in which two light fluxes having different wavelengths are radiated on the same light path so that the light sources are used in common (see, for example, Patent Document 2). When the ( 1/7) wavelength plate shown in Patent Document 1 is provided in this twin beam optical system, the apparatus must adapt to the two wavelengths of a DVD and a CD. Moreover, it is known as another problem that the operating range of the wavelength plate made of a material using such a crystal is small.    Patent Document 1: JP-B2-3545008    Patent Document 2: JP-A-2005-141849
Non-Patent Document 1: “Structural double refraction and application to optical element” of Introduction to Diffraction Optical Element written by Hisao Kikuta belonging to the optical design group of the Optical Society of Japan of the Japan Society of Applied Physics and published by The Optronics Co., Ltd., on May 20, 1998, first edition, p 158 to p 167
Non-Patent Document 2: “12.7 mm slim type BD drive” in Panasonic Technical Journal, Vol. 54, No. 3 written by Shogo Horinouchi, Shohei Yumida, Tomohiro Matsuo and published by Panasonic Corporation on Oct. 28, 2008, p 15 to p 20